[Haskell-cafe] Re: Why are these record accesses ambiguous

Hi John, The record field disambiguation only works if you use the form

C{ field-name = variable }

where C is a datatype constructor. In your example you have to write

let TypeA{ x = v } = getA print v

You're right, after type inference it is clear (for us) that x should mean A.x, but this kind of reasoning (disambiguate names based on the results of type inference) is not supported by ghc - and that's a good thing, in my opinion, as otherwise it would be incredibly hard to find the definition in scope. There was a long thread on cafe on this subject. cheers, benedikt John Ky schrieb:

Hi Luke,

You're right. My code had a typo. Unfortunately, I still get the same error whichever way I do it.

For example:

...

{-# LANGUAGE DisambiguateRecordFields #-} import A import B

main = do let xx = getA print (x xx)

and:

#!/usr/bin/env runhaskell

...

{-# LANGUAGE DisambiguateRecordFields #-} import A import B

main = do let xx = getA putStrLn $ show (x xx)

both give me:

test.lhs:8:22: Ambiguous occurrence `x' It could refer to either `A.x', imported from A at test.lhs:3:2-9 (defined at A.hs:5:5) or `B.x', imported from B at test.lhs:4:2-9 (defined at B.hs:5:5)

Any ideas?

$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.10.3

Thanks,

-John

On Sat, Jun 6, 2009 at 6:41 PM, Luke Palmer mailto:lrpalmer@gmail.com> wrote:

On Sat, Jun 6, 2009 at 1:48 AM, John Ky mailto:newhoggy@gmail.com> wrote:

Hi Haskell Cafe,

In the following code, I get an error saying Ambiguous occurrence `x'. Why can't Haskell work out which x to call based on the type of getA?

Thanks

-John

#!/usr/bin/env runhaskell

> {-# LANGUAGE DisambiguateRecordFields #-} > import A > import B > > main = do > let xx = getA > putStrLn $ show x xx

This is parsed as two arguments passed to the show function (which only takes one argument).

putStrLn $ show (x xx)

Or because putStrLn . show = print;

print $ x xx

----------------------

module A where

data TypeA = TypeA { a :: Int , x :: Int }

getA = TypeA { a = 1, x = 2 }

-------------------------

module B where

data TypeB = TypeB { b :: Int , x :: Int }

getB = TypeB { b = 1, x = 3 }

--------------------------

./test.lhs:8:21: Ambiguous occurrence `x' It could refer to either `A.x', imported from A at ./test.lhs:3:2-9 (defined at A.hs:5:5) or `B.x', imported from B at ./test.lhs:4:2-9 (defined at B.hs:5:5)

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